In general, an automobile steel wheel, which is manufactured by performing press working on a metal plate, includes a wheel rim configured to retain a tire, and a wheel disk to be coupled to an axle hub. The wheel rim and the wheel disk are formed integrally with each other by welding the wheel rim on an outer circumference of the wheel disk. A hub coupling portion being a region to be coupled to the axle hub is formed at a center of the wheel disk. A hub hole and bolt holes arranged around the hub hole at equal intervals are formed in the hub coupling portion. The automobile wheel is fastened to the axle hub in such a manner that nuts are fastened to stud bolts fixed to the axle hub under a state in which a cylindrical portion formed at a distal end of the axle hub is inserted through the hub hole and the stud bolts are inserted through the bolt holes.
A hub cap (also referred to as a hub ornament) is mounted to the hub coupling portion from a front surface side of the wheel disk so as to cover the hub hole. The cylindrical portion of the axle hub can be covered with the hub cap. For example, in Patent Literature 1, a technology of mounting the hub cap is proposed. An automobile wheel proposed in Patent Literature 1 has such a configuration that a hub hole is formed into a toothed gear shape so that gaps are defined intermittently between a cylindrical portion of an axle hub and a peripheral edge of the hub hole. Further, coupling protruding portions of the hub cap are pushed into the gaps, thereby fixing the hub cap. In the automobile wheel, cap fixing grooves are formed intermittently along a circumferential direction in a back surface of a hub hole peripheral portion surrounding the hub hole, and the coupling protruding portions of the hub cap are fitted into the cap fixing grooves, respectively.
In order to form the cap fixing grooves in the hub hole peripheral portion in the automobile wheel proposed in Patent Literature 1, two compressing steps (a first compressing step and a second compressing step) and a cutting step are performed. In the first compressing step, punching of the hub hole and basic formation of the cap fixing grooves are performed. In the basic formation, grooves shallower than the finished cap fixing grooves are formed. In the second compressing step, compressing is performed using a drawing die and a forming punch, thereby increasing depths of the grooves formed by the basic formation. As a result, the cap fixing grooves each having a predetermined depth (depth of a finished shape) are formed. In the second compressing step, a gap is provided for discharging an excess material, which is generated at the time of the compressing, radially inward (to a hub hole center side), and the excess material is extruded into the gap. In the cutting step, the excess material extruded radially inward is cut using a hub hole trimming die and a hub hole trimming punch. In this manner, the hub hole is formed, around which the cap fixing grooves are formed.